Canadian Patent No 1,241,211 dated Aug. 30, 1988 entitled: "LOCKING DEVICE FOR INTERLOCKING NESTED SHOPPING CARTS", inventor Antoine Truliano, describes a housing attachable to the handle bar of a shopping cart and provided with a locking mechanism to retain a key inserted in a key slot of the housing. To unlock the key, a coin must be inserted in a coin-receiving actuator, which is pushed inside the housing, thereby retaining the coin therein. The key is attached by a flexible link, such as a chain, to the housing of an adjacent cart, so that several carts can be retained interlocked in nested position. A disadvantage of this interlocking system resides in the fact that the effective length of the chain attaching the key to a housing must be shorter than that required to permit insertion in the key slot of the same housing of the key attached to latter. If the chain was long enough to permit this key insertion, then it would be possible for the shopper to retrieve the coin inserted in the housing by simply inserting the key carried by the housing of his cart into the key slot of this housing, thus avoiding having to return the cart to the row of nested carts for the insertion of the key of the next cart for coin retrieval and locking of the just-returned cart to the next cart. Therefore, the distance between the handle bars of nested carts must be somewhat less than the above defined maximum length of the key chain. This is not always possible to obtain because due to the large tolerances allowed in their construction, carts even of the same make do not always nest to the same extent. Also it has been found that the distance between the handle bars of nested carts of various makes can vary between 9" and 18". It follows that the same model of locking device in accordance with the patent cannot be used for different makes of nestable carts.
Another problem resides in the fact that the key freely hangs from the housing attached to the cart being used, and this key can be a source of accidents especially for children. A holder is provided for retaining the free key, but this is not always used.
Another disadvantage of the patented device resides in the complexity of its mechanism.